The present invention relates generally to the field of mechanical power controls, and more particularly to the field of overload prevention devices.
It has long been recognized that some cutoff means must be provided for stopping the application of torque through a power train, lest the power source continue trying to apply torque when the system cannot perform further rotation. The conventional approach to that problem is the provision of limit switches, positioned at the end of a normal range of travel in a system, to cut off motive power when the rotational element reaches that point. Such an arrangement is particularly common in systems where a power-driven rotating element is not positioned for easy service access.
An example of such a system is a damper actuator, found in widespread use in building heating, ventilating and air conditioning systems. There, an air supply duct is regulated by a damper, driven by a power source. Pneumatic power sources, in the past the most common type, presented no significant design problem, as such systems were easily controllable through pressure relief valves. Modern systems, however, derive motive power from electric motors, which apply torque to a damper shaft through a reduction gear train, typically at high torque levels.
The conventional solution of limit switches has been applied to such systems. That design, however, suffers from a serious limitation, as it assumes that the only occasion for removing power from the motor arises at the end of normal damper travel. That assumption ignores the possibility of an obstruction in the system, caused by a foreign object wedged in the damper itself, or some minor component failure. In that event, the motor will continue applying torque to the system until the damper or its shaft breaks, or until the motor burns out. In either event, the failure to provide adequate protection turns a simple problem into a complicated and expensive one.
What has been lacking is some means for sensing a torque overload and for removing power when that condition is present, regardless of the damper position. That lack is met in the present invention.